1. Field of the Invention
The invention relates generally to a downhole formation analysis and in particular to demagnetizing a borehole.
2. Summary of the Related Art
It is well known that drilling mud may contain magnetic particles which may influence surveys taken by monitoring while drilling (MWD) directional sensors. This effect is described in IADC/SPE 87169 and SPE 71400. The particles are mainly from casing and drill-string wear. The particles collect especially in synthetic oil-based mud which is recycled and used for a long period of time and even at different locations. Magnetic filters (ditch magnets) can not filter the magnetic particles out of the mud because they are too small to be attracted to the magnetic filter. The mud contaminate with magnetic steel particles can invade the formation and can also collect in the filter cake. This collection of magnetic particles creates a zone close to the borehole wall containing magnetic material which can not circulate with the rest of the drilling mud. When permanent magnets from nuclear magnetic resonance (NMR) tools, magnetic fishing tools or casing collar locaters are moved through the borehole, these collected particles in the filter cake or the invaded zone can become magnetized. Because of the magnetic resonance the particles maintain their magnetic influence and emit a magnetic field for a long time. The magnetic field created by these magnetically aligned particles can influence the performance of magnetic tools deployed from a wireline or drill string such as magnetic azimuth measurements or other magnetic measurements associated with the borehole. A similar problem occurs in cased borehole where the casing is made of a magnetic material such as steel. The resulting magnetization of the casing may affect the performance of sensors such as magnetometers that are subsequently conveyed through the casing.